Human cytochrome P450 enzymes are responsible for >90% of drug metabolism and synthesis of important endogenous compounds including hormones. Our current structural knowledge of these membrane enzymes is based on X-ray structures, but we understand little of the enzymes' behavior in solution or with ligands that do not form tight complexes amenable to crystallization. Therefore, important structural aspects of human P450 enzyme function have not been monitored in solution. Our long-term goal is to introduce and exploit solution NMR as a tool to interrogate P450 function at the atomic level in solution. Our immediate objective is to apply 2D solution NMR to study ligand binding in the human androgen-producing cytochrome P450 17A1 (CYP17A1) to direct drug design efforts. Since >240,000 U.S. men will be diagnosed with prostate cancer this year and prostate cancer proliferates in response to androgens, inhibiting cytochrome P450 (CYP17A1) is an effective way to treat metastatic prostate cancer. However, CYP17A1 performs two enzymatic reactions (hydroxylase and lyase) and inhibition of only one of them is desirable for prostate cancer treatment without serious side effects. The search for reaction-selective CYP17A1 inhibitors is limited by a lack of understanding regarding how the enzyme functions. Although we produced an X-ray crystal structure of CYP17A1 bound to a new drug inhibitor of both reactions (abiraterone), crystals with the natural substrates of CYP17A1 for each reaction have proven elusive. In this study, we will use 2D Nuclear Magnetic Resonance (NMR) to generate detailed information regarding the location and orientation of both natural substrates and new inhibitors in the active site, guiding the subsequent development of lyase-selective inhibitors for prostate cancer treatment. Our central hypothesis is that different substrates bind in the same general location and orientation as the steroidal inhibitor abiraterone, but with minor differences accounting for their participation in the hydroxylase vs. lyase reactions. We will test this hypothesis using two specific aims: 1) comparing the location and orientation of hydroxylase substrates with abiraterone; and 2) identifying the location and orientation of lyase substrates and a lyase-only inhibitor. Previous application of NMR to study membrane cytochrome P450 enzymes has been limited by technical obstacles related to their relatively large size (55-60 kDa), hydrophobic membrane-binding surfaces, and challenging over-expression in minimal media needed for isotopic labeling. We have overcome these to produce sufficient quantities of isotopically labeled protein, determine conditions to maintain membrane protein solubility while collecting NMR data, and generate selectively labeled CYP17A1 to simplify the NMR spectrum of this 55.6 kDa enzyme. These successes permit, for the first time, investigation of human P450 enzymes by multidimensional NMR. Therefore this work not only directly informs the design of novel CYP17A1 inhibitors, but also provides the groundwork to use this approach for many other human cytochrome P450 enzymes, likely impacting the future study of human drug metabolism.